Breaking Bad protagonist Walter White was so introverted he "imploded", leading to his dramatic transformation, Bryan Cranston has said.

The actor, who has portrayed Walter in Vince Gilligan's drama since 2008, told The Guardian that the character's shocking acts were triggered by a rush of emotion that he experienced when he learned he was dying of lung cancer.

He explained: "In my initial research for a role, I always try to find the emotional core of a character. For Hal in Malcolm in the Middle, it was fear. He was afraid of everything and there's a lot of comedy to be mined from fear.

"With Walt, I had a difficult time. I was trying to get in there and I was getting frustrated. Then I realised: 'Oh, I know what it is.'

"In the beginning, his emotional core was so introverted from looking at apathetic students and his own missed opportunities that he imploded. He loved his family but for the most part it was just about getting by. Then this thing happened, this diagnosis, and a volcano erupted."

Although Walter's actions become increasingly violent and outrageous, Cranston said that his original decision to start making crystal meth sets in motion a series of events that he feels are justified.

He said: "The thing that feels the most telling, really, is the very first decision when Walter chose to become someone he's not. The first time he decided, 'I'm going to cook crystal meth and make money.' That's not him. He's not in his element.

"For a smart man, it was a stupid, very shortsighted decision. He soon realises he doesn't have the skills for it, but by now it's rolling, and it's rolling away from him. It's like an avalanche: he's constantly just ahead and he's got to keep moving. So when it comes to later in the seasons and he calls for the execution and murder of nine people in jail it actually makes sense to him."